Many of plastics derived from petroleum are light-weight, tough, superior in durability and are capable of being easily formed in an arbitrary shape, and thus have been mass-produced to support our life in various fields. However, these plastics are not decomposed easily and accumulate when disposed in environment. In addition, incineration thereof emits a large quantity of carbon dioxide, which has accelerated global warming.
In consideration of such a present state, a resin composed of a non-petroleum-based raw material, or a biodegradable plastic decomposable by a microorganism has been studied actively. Most of the biodegradable plastics presently studied have an aliphatic carboxylate ester unit, and are easily decomposed of a microorganism. On the other hand, they are inferior in heat stability, and have a serious problem of decrease in molecular weight or hue deterioration in a molding process, wherein they are exposed to high temperature, such as melt spinning, injection molding, melt film formation or the like.
Among these, polylactic acid (hereafter may be referred to as PLA) is a plastic superior in heat resistance and has good balance of hue and mechanical strength. As a method for producing a homo-polymer (hereafter may be referred to as homo-PLA) of poly-L-lactic acid (hereafter may be referred to as PLLA) or poly-D-lactic acid (hereafter may be referred to as PDLA), a ring-opening polymerization method using lactide as a raw material or a direct polymerization method by dehydration condensation of lactic acid is known. In general, melting point of the above-described poly-L-lactic acid and the above-described poly-D-lactic acid is said to be around 170° C. Therefore, they have lower heat resistance as compared with a petrochemistry-based polyester represented by polyethylene terephthalate or polybutylene terephthalate, and have a problem that, for example, ironing is not applicable onto a product, in the case where thus produced poly-L-lactic acid is used as a fiber or the like. Therefore, it is a present state that further higher heat resistance is required.
In an attempt to find a breakthrough in such a state, improvement of heat resistance of polylactic acid has been studied. One of these studies includes a stereo-complex polylactic acid (hereafter may be referred to as scPLA) formed by mixing poly-L-lactic acid and poly-D-lactic acid.
However, in the case where poly-L-lactic acid and poly-D-lactic acid are mixed in a mass ratio of 1:1, only a stereo-complex crystal does not always appear, and in many cases, a homo-PLA crystal also appears at the same time, in particular, in a region of high molecular weight. In addition, it is difficult to stably form scPLA having 100% rate of content of a stereo-complex crystal by heat melting process, and such a presence of homo-PLA raised a problem of insufficient utilization of features of scPLA having high melting point. Therefore, it is strongly required to stably produce a product having 100% rate of content of scPLA, without mixed homo-PLA.
Therefore, as an alternative method for producing scPLA by mixing the above poly-L-lactic acid and poly-D-lactic acid, a method is proposed for producing a polylactic acid block copolymer, wherein the same amount of poly-L-lactic acid and poly-D-lactic acid are mixed, and the poly-L-lactic acid and poly-D-lactic acid are subjected to a reaction to form a covalent bond (for example, see JP-A-2002-356543). By this method, scPLA is preferentially generated between a chain of L-lactic acid units and a chain of D-lactic acid units, among molecules of polylactic acid block copolymers, and thus a peak derived from homo-PLA is not confirmed on a chart of a differential scanning calorimetry (DSC). Namely, a product with 100% rate of content of scPLA can be stably formed, without mixed homo-PLA.
In addition, in JP-A-2003-342836, a heat adhesive fiber is disclosed, which is a core-sheath conjugated type fiber of polylactic acid-based polymer, forming a stereo-complex, wherein a component composing the core part is obtained by blending poly-L-lactic acid having an optical purity of from 70 to 100% ee, and poly-D-lactic acid having an optical purity of from 70 to 100% ee.